1. Field of the Invention
This invention relates generally to active hybrid circuits, and more particularly to an active R-C integrated circuit that enhances modem performance without compromising its hybrid rejection while avoiding the use of inductors.
2. Description of the Prior Art
Signals to and from a modem coexist simultaneously; and both the transmitted and received signals are superposed at nearly all circuit nodes in a full-duplexed communication system. Circuit techniques must then be employed to eliminate the transmitted signal from the receiver signal path to avoid corrupting the signal-to-noise ratio associated with the received signal. When the signals occupy different portions of the frequency spectrum, this can be achieved by using filters. If however, their spectra overlap, a circuit commonly referred to as a hybrid must be employed.
FIG. 1 illustrates a general hybrid 102 used in a communication system 100. Nodes A 104 and B 106 comprise different, generally frequency-dependent linear combinations of signals VT and VR. If the communication medium 108 can be modeled as a resistive element, RM, then                               V          B                =                                                            V                R                            ⁡                              (                                  R                  S                                )                                      +                                          V                T                            ⁡                              (                                                      R                    M                                    +                                      R                    L                                                  )                                                                        R              S                        +                          R              M                        +                          R              L                                                          (        1        )            
The transmitted signal may be eliminated in the receiver path 110 if                               V          H                =                              -                                          V                A                            ⁡                              (                                                      R                    M                                    +                                      R                    L                                                  )                                                                        R              S                        +                          R              M                        +                          R              L                                                          (        2        )            
such that the receiver path signal becomes VB+VH. Thus, the hybrid 102 acts as an inverting voltage divider in this case, and as a frequency-dependent transfer function (filter) in general. The scheme 100 works whenever the linear combinations of VT and VR at nodes A 104 and B 106 are different.
Hybrid rejection is a measure of the degree to which the VT component is eliminated from the receiver path 110. Complete elimination is rarely ever achieved largely because it would require unacceptably complex hybrid circuits to replicate the voltage division that takes place when the communication medium 108 consists of transmission lines and transformers.
Hybrids to date have employed passive circuits. When transformers are part of the transmission medium 108, and particularly when the transformer winding inductance is small, good hybrid rejection at low frequencies requires that one or more inductors be used in the hybrid circuit 102. This is problematic since the required inductors undesirably increase the hybrid parts count as well as the area of the modem printed circuit board that employs the hybrid 102.
It is therefore both advantageous and desirable in view of the foregoing, to provide a hybrid circuit that is suitable for use with a modem associated with a communication medium comprising transformers, but that does not compromise its hybrid rejection while avoiding the use of inductors.
The present invention is directed to a hybrid circuit that is suitable for use with a modem associated with a communication medium comprising transformers, but that does not compromise its hybrid rejection while avoiding the use of inductors. The hybrid transfer function has three zeros and four poles that are realized using only two fully-differential amplifiers in combination with a small plurality of resistors and capacitors, making the hybrid suitable for use with a communication medium comprising capacitively coupled non-ideal transformers and transmission lines while providing remarkably good hybrid rejection without the use of inductors.
According to one embodiment, an active R-C circuit comprises a fully-differential amplifier; a pair of single-ended buffers; and a resistor-capacitor network connecting the fully-differential amplifier and the pair of single-ended buffers, wherein the filly-differential amplifier, pair of single-ended buffers and resistor-capacitor network are configured as a hybrid devoid of inductors to realize a transfer function associated with substantial elimination of a receiver path signal associated with a communication medium comprising capacitively coupled non-ideal transformers and transmission lines.
According to another embodiment, an active R-C circuit comprises a pair of fully-differential amplifiers; and a resistor-capacitor network connecting the pair of fully-differential amplifiers to realize a transfer function associated with substantial elimination of a receiver path signal associated with a communication medium comprising capacitively coupled non-ideal transformers and transmission lines, wherein the pair of fully-differential amplifiers and the resistor-capacitor network are configured as a hybrid devoid of inductors.
According to yet another embodiment, a method of providing a desired hybrid rejection comprises the steps of providing an active R-C circuit comprising a fully-differential amplifier, a pair of single-ended buffers; and a resistor-capacitor network connecting the fully-differential amplifier and the pair of single-ended buffers such that the fully-differential amplifier, pair of single-ended buffers and the resistor-capacitor network realize a hybrid transfer function consisting of three zeros and four poles; connecting the active R-C circuit into a communication system that employs a communication medium comprising transformers; and processing a transmitted signal passing through the active R-C circuit such that the processed signal is combined with a second signal associated with the communication system to substantially prevent the transmitted signal from passing through a communication system receiver path to provide substantially an equal level of hybrid rejection as that provided by a hybrid circuit that employs inductors to achieve a desired hybrid rejection associated with a communication medium comprising capacitively coupled non-ideal transformers and transmission lines.
According to still another embodiment, a method of providing a desired hybrid rejection comprises the steps of providing an active R-C circuit comprising a pair of fully-differential amplifiers and a resistor-capacitor network connecting the pair of fully-differential amplifiers to realize a hybrid transfer function consisting of three zeros and four poles; connecting the active R-C circuit into a communication system that employs a communication medium comprising transformers; processing a transmitted signal passing through the active R-C circuit such that the processed signal is combined with a second signal associated with the communication system to substantially prevent the transmitted signal from passing through a communication system receiver path to provide substantially an equal level of hybrid rejection as that provided by a hybrid circuit that employs inductors to achieve a desired hybrid rejection associated with a communication medium comprising capacitively coupled non-ideal transformers and transmission lines.
The particular embodiments of the active R-C circuit comprise a transfer function that can optionally be made programmable via a data processing device such as a digital signal processor, a central processing unit, a computer, a micro-computer, and a micro-controller, among others.